Fig. 1

a Tectonic background and b events and stations used in this study. Green, yellow, and sky blue stars indicate the epicenters of the largest foreshock (M _JMA6.5), the mainshock (M _JMA7.3), and the induced event (M _JMA5.7), respectively. White stars denote the epicenters of events larger than M _JMA 5.0 for one month following the foreshock. Black circles indicate the epicenters of events with 2.0 ≤ M _JMA < 5.0 occurring for one month following the foreshock. Active and quaternary volcanoes are shown as red and peach triangles, respectively. Purple lines denote active faults (Nakata and Imaizumi 2002). Black lines are contours of altitude with intervals of 300 m. The red square corresponds to the area shown in Fig. 2. Colored circles and inverse triangles show the events and stations used in this study. The color and size of each circle indicate the depth and the magnitude of the event, respectively